Electrochemical cells comprising ceramic membranes that selectively transport ions are known in the art. By having an ion-selective membrane in the electrochemical cell, certain ions are allowed to pass between the cell's anolyte compartment and catholyte compartment while other chemicals are maintained in their original compartments. Thus, through the use of an ion-specific membrane, an electrochemical cell can be engineered to be more efficient and to produce different chemical reactions than would otherwise occur without the membrane.
These ion-selective membranes can be selective to either anions or cations. Moreover, some cation-selective membranes are capable of selectively transporting alkali cations. By way of example, NaSICON (Na Super Ion CONducting) membranes selectively transport sodium cations, while LiSICON (Li Super Ion CONducting) and KSICON (K Super Ion CONducting) membranes selectively transport lithium and potassium cations, respectively.
One example of a conventional electrochemical cell is illustrated in FIG. 1. Specifically, FIG. 1 illustrates an electrolytic cell 110 that comprises an anolyte compartment 112 and a catholyte compartment 114 that are separated by a NaSICON membrane 116.
Under some conventional methods, as the cell 110 operates, the anolyte compartment 112 comprises an aqueous alkali (e.g. sodium) salt solution (NaX, wherein X comprises an anion capable of combining with a sodium cation to form a salt) and current is passed between an anode 118 and a cathode 120. Additionally, FIG. 1 shows that as the cell 110 operates, water (H2O) can be split at the anode 118 to form oxygen gas (O2) and protons (H+) through the reaction 2H2O→O2+4H++4e−. FIG. 1 further shows that the sodium salt NaX in the anolyte solution can be split (according to the reaction NaX+H+→HX+Na+) to: (a) allow sodium cations (Na+) to be transported through the NaSICON membrane 116 into the catholyte compartment 114 and (b) to allow anions (X−) to combine with protons to form an acid (HX) that corresponds to the original sodium salt. Similarly, FIG. 1 shows that as the cell 110 operates, water (H2O) can be split at the cathode 220 to form hydrogen gas (H2) and hydroxyl ions (OH−) through the reaction 2H2O+2e−→H2+2OH−. FIG. 1 further shows that the sodium cations transported through the NaSICON membrane 116 can combine with hydroxyl ions in the catholyte solution according to the reaction OH−+Na+→NaOH.
As electrochemical cells operate with the alkali ion-selective membrane exposed to adverse conditions, some such cells may have shortcomings. In one example, at a lower pH, such as a pH less than about 5, certain alkali ion-conductive ceramic membranes, such as NaSICON membranes, may become less efficient or unable to transport alkali cations. Accordingly, as the electrochemical cell operates and acid is produced in the anolyte compartment, the cell may become less efficient or even inoperable. In another example, acid produced in the anolyte compartment can actually damage the alkali ion-selective membrane, such as a NaSICON membrane, and thereby shorten its useful lifespan.
In other examples, electrochemical cells may be operated using catholyte and/or anolyte solutions (such as basic solutions; organic solutions; neutral solutions containing a detrimental ion, such as potassium, lithium or cesium, that effects membrane efficiency; etc.) that are chemically reactive to, or that otherwise damage or reduce the efficiency of, the alkali ion-conductive electrolyte membrane. While such solutions may be added directly to the cell, in some instances, the solutions are generated as the cell operates. For instance, where the cell comprises an organic solvent (e.g., ethylene glycol, hexanol, etc.), operation of the cell, especially at high voltages and for long periods of time) may cause the organic solvent to react and form a resistive film on the membrane and, thereby, reduce the cell's overall efficiency. Additionally, in some instances in which the cell comprises an organic compound (e.g., methanol), the organic compound evolves protons as the cell functions, which, in turn, can lower the pH of the solvent contacting the membrane. In still other instances in which the cell (e.g., a battery) comprises a fluorinated compound (e.g., LiPF6) and even a trace amount of water, the cell may function to produce hydrofluoric acid (HF), and thereby reduce the pH of the materials in contact with the membrane. As discussed above, this reduction in the pH of materials contacting the membrane can cause the cell to be less efficient or even inoperable.
In yet other examples, electrochemical cells may be operated using molten metals, such as molten anode or cathode materials which may be chemically reactive to the alkali ion-conductive electrolyte membrane. In still other examples, the electrolytic cells may be operated using molten salts which may be chemically reactive to the alkali ion-conductive electrolyte membrane.
Thus, while electrochemical cells comprising a catholyte compartment and an anolyte compartment that are separated by an alkali ion-conductive membrane are known, challenges still exist, including those mentioned above. Accordingly, it would be an improvement in the art to augment or even replace current electrochemical cells with other cells or methods for using the cells. More specifically, it would be an improvement in the art to protect the alkali ion-conductive electrolyte membrane from undesired chemical reactions and thereby maintain its alkali ion conductivity.